A known fuel-lubricated pump P1 adapted to supply fuel to a fuel injection system for an internal combustion engine is shown in FIG. 1. The fuel-lubricated pump P1 relies on the fuel being pumped as a lubricating medium. The fuel-lubricated pump P1 is configured to supply diesel fuel at high pressure to a common rail (not shown) connected to a plurality of fuel injectors. The fuel-lubricated pump P1 comprises a hydraulic head assembly 101 disposed in a pump housing H1. As shown in FIG. 2, the hydraulic head assembly 101 comprises a hydraulic head 102 having a body 103 and a turret 104. The turret 104 comprises a cylindrical projection formed integrally with the body 103. A first bore 105 and a pumping chamber 106 are formed in the hydraulic head 102. The first bore 105 has a longitudinal axis X and extends through the turret 104. The pumping chamber 106 has a larger diameter than the first bore 105. A pumping element 107 in the form of a plunger is disposed in said first bore 105 to pressurise fuel in the pumping chamber 106. The pumping element 107 cooperates with a rotating cam via a roller shoe assembly to cause the pumping element 107 to reciprocate within the first bore 105. An inlet valve 108 is provided for controlling the introduction of fuel into the pumping chamber 106. The inlet valve 108 comprises an inlet valve member 109 arranged to cooperate with an inlet valve seat 110. A first spring element 111 cooperates with the inlet valve member 109. An outlet valve 112 is provided for controlling the expulsion of fuel from the pumping chamber 106 to the common rail. The outlet valve 112 comprises an outlet valve member 113 arranged to cooperate with an outlet valve seat 114. A second spring element 115 is provided to bias the outlet valve member 113 into a seated position in said outlet valve seat 114, thereby closing the outlet valve 112. The outlet valve 112 is opened when the pressure of the fuel in the pumping chamber 106 overcomes the spring bias applied to the outlet valve member 113 by the second spring element 115 and the fuel pressure in the common rail. The first bore 105 and the pumping element 107 are sized to form a first seal 116. The first seal 116 is a high pressure seal and has a high pressure leakage control length L2 which is extended by the turret 104.
A limitation of the fuel-lubricated pump P1 is a potential lack of robustness to poor lubricity fuels, for example due to different grades of fuel available in different territories. A further potential limitation is in the length of the high pressure leakage control length L2 due to the need accurately to machine the inlet valve seat 110 over the length L1.
It is known to provide a fuel pump with a lubricating medium, typically oil, which is maintained separate from the fuel. A known oil-lubricated pump P2 is shown in FIG. 3. The oil-lubricated pump P2 is configured to supply diesel fuel at high pressure to a common rail (not shown) connected to a plurality of fuel injectors. The oil-lubricated pump P2 comprises a hydraulic head assembly 201 disposed in a pump housing H2. As shown in FIG. 4, the hydraulic head assembly 201 comprises a hydraulic head 202 having a body 203 and a turret 204. A first bore 205 and a pumping chamber 206 are formed in the hydraulic head 202. The pumping chamber 206 has a larger diameter than the first bore 205. A pumping element 207 in the form of a plunger is disposed in said first bore 205 to pressurise fuel in the pumping chamber 206. The pumping element 207 cooperates with a rotating cam via a roller shoe assembly (not shown) to cause the pumping element 207 to reciprocate within the first bore 205. An inlet valve 208 is provided for controlling the introduction of fuel into the pumping chamber 206. The inlet valve 208 comprises an inlet valve member (not shown) arranged to cooperate with an inlet valve seat 210. An outlet valve 212 is provided for controlling the expulsion of fuel from the pumping chamber 206 to the common rail. The outlet valve 212 comprises an outlet valve member 213 arranged to cooperate with an outlet valve seat 214. The first bore 205 and the pumping element 207 are sized to form a first seal 216 and a second seal 217. The first seal 216 is a high pressure seal and has a high pressure leakage control length L2. The second seal 217 is a low pressure seal and has a low pressure leakage control length L3. A third seal 218 in the form of a polytetrafluoroethylene (PTFE) lip seal is disposed below the second seal 217. An annular chamber 219 and a return line 220 are formed in the hydraulic head 202 between the first and second seals 216, 217. The second seal 217 functions as a back-up seal for the third seal 218. The oil-lubricated pump P2 can be assembled to form a direct unit pump (DUP) which is located directly into an engine block. The hydraulic head assembly 201 can also be disposed in a cam box to form an oil lubricated pump, as shown in FIG. 3.
The high pressure leakage control length L2 of the first seal 216 must be sufficient to maintain pumping efficiency; and the low pressure leakage control length L3 must be of sufficient length to inhibit mixing of oil into the fuel. However, the need accurately to machine the inlet valve seat 210 restricts the high and low pressure leakage control lengths L2, L3 of the first and second seals 216, 217.
The oil-lubricated pump P2 provides improved robustness to different grades of fuel. Nonetheless, production volumes of oil-lubricated pumps are expected to remain relatively low. The lower production volumes reduce the economies of scale for production of the oil-lubricated pump P2. This is particularly problematic in view of the need for a specific design of hydraulic head 202.
It is against this backdrop that the present invention has been conceived. At least in certain embodiments, the present invention seeks to overcome or ameliorate at least some of the problems associated with known fuel pumps.